Protocol for doping of an Sn-based two-dimensional perovskite semiconductor by incorporating SnI4 for field-effect transistors and thermoelectric devices

Summary Doping is an important technique for semiconductor materials, yet effective and controllable doping of organic-inorganic halide perovskites is still a challenge. Here, we present a protocol to dope 2D perovskite (PEA)2SnI4 by incorporating SnI4 in the precursor solutions. We detail steps for preparation of field-effect transistors (FETs) and thermoelectric devices (TEs) based on SnI4-doped (PEA)2SnI4 films. We further describe characterization via conductivity measurement using the four-point probe method, FETs performance, and TEs performance measurements. For complete details on the use and execution of this protocol, please refer to Liu et al. (2022).1


SUMMARY
Doping is an important technique for semiconductor materials, yet effective and controllable doping of organic-inorganic halide perovskites is still a challenge. Here, we present a protocol to dope 2D perovskite (PEA) 2 SnI 4 by incorporating SnI 4 in the precursor solutions. We detail steps for preparation of field-effect transistors (FETs) and thermoelectric devices (TEs) based on SnI 4 -doped (PEA) 2 SnI 4 films. We further describe characterization via conductivity measurement using the four-point probe method, FETs performance, and TEs performance measurements. For complete details on the use and execution of this protocol, please refer to Liu et al. (2022). 1

Timing: 2-4 h
The protocol below describes the specific steps for preparing and characterizing field-effect transistors (FETs) and thermoelectric devices (TEs) based on SnI 4 -doped (PEA) 2 SnI 4 films. 1. Check the oxygen and moisture level in the glovebox, it should be approximately or less than 0. The preparation process includes preparing perovskite precursor solutions and spin-coating precursor solutions on cleaned substrates.

Bottom-contact electrodes fabrication.
Define and deposit the bottom-contact electrodes by photolithography and thermal evaporator (< 4 3 10 -4 Pa), respectively. a. Define bottom-contact electrodes by photolithography. i. Dissolve 2 g NaOH with 500 mL deionized water as developer.
ii. Spin the positive photoresist on the desired 4-inch wafer substrate with speed of 3,000 rpm for 30 s and anneal at 105 C for 5 min. iii. Expose the substrate under UV light through shadow mask for 7 s. iv. Soak the substrate into the developer for 10 s, and dry them with nitrogen gas gun. b. For conductivity measurement, Figure 1A shows the electrode structure. Deposit the Cr (2 nm) and Au (30 nm) sequentially on Si ++ /SiO 2 substrates by a thermal evaporator. c. For FET devices, Figure 1B shows the electrode structure. Deposit Cr (2 nm) and Au (30 nm) sequentially on Si ++ /SiO 2 substrates by a thermal evaporator. d. For TE devices, Figure 2A shows the electrode structure. Deposit Cr (10 nm) and Au (15 nm) sequentially on glass substrates by a thermal evaporator. e. Soak the substrates into acetone for 30 min to lift off the photoresist, and dry them with nitrogen gas gun.
Note: For Si ++ /SiO 2 substrates, Si ++ refers to heavily doped silicon with low resistivity of 0.01 U/cm. The thickness of SiO 2 is 300 nm.   ii. Treat the substrates with UV/ozone for 30 min before spin-coating.
iii. Cast 30 mL precursor solution in the center of the Si ++ /SiO 2 substrate and spin-coat at 4,000 rpm for 30 s with acceleration of 500 rpm/s. iv. Anneal the perovskite films at 100 C for 10 min. c. For TEs measurement, pattern the perovskite films as shown in Figure 2.
i. Ultrasonicate the glass substrates sequentially in deionized water, acetone and isopropanol for 2 min each. ii. Blow dry them by a nitrogen gas gun. iii. Treat the substrates with UV/ozone for 30 min. iv. Spin Octadecyltrichlorosilane (ODTS) solution (5 vol& in toluene) on the glass substrates with at 2,000 rpm for 30 s with acceleration of 1,000 rpm/s.

OPEN ACCESS
v. Anneal the substrates at 100 C for 10 min to form an ODTS film on substrate ( Figure 2B). vi. Cover the area except for the hot and cold ends with tape. vii. Treat the substrates with UV/ozone for 30 min ( Figure 2C) before removing the tape ( Figure 2D). viii. Cast 10 mL precursor solution in the center of the glass substrate and spin-coat at 4,000 rpm for 30 s with acceleration of 500 rpm/s. ix. Anneal the perovskite films at 100 C for 10 min to form patterned perovskite film ( Figure 2E). The characterizations include conductivity, FETs performance, and TEs performance measurements. Due to the instability of perovskite films, conduct all measurements immediately in Ar-filled glovebox in dark after preparation unless otherwise stated.

Conductivity measurement.
Measure the conductivity of perovskite film by four-point probe method, Figure 1A shows the electrode structure.
a. Connect the four electrodes with a Keithley 4200 semiconductor analyzer using a probe station. b. As shown in Figure 1A, the voltages of the four electrodes are named V 1 (= 0 V), V 2 , V 3 and V 4 , respectively. c. After the four-point probe measurement, identify the thicknesses of samples by atomic force microscopy (AFM).
CRITICAL: The measured V 4 should be more than 1 V and less than 20 V by adjusting the maximum value of sweep current I. Figure 1B  ii. Set the V GS as 0, -20 and -40 V, respectively.

FETs measurement.
CRITICAL: Carry out all measurement processes under dark conditions. CRITICAL: Conduct the transfer and output characteristics twice, and save the result of the second measurement, because the light exposure before the test will affect the results of the first measurement.
To measure the Seebeck coefficients of doped (PEA) 2 SnI 4 films, we use a homemade thermoelectric measurement system, as shown in Figure 2A. 2 a. Connect the pads of on-chip stripe heater, hot end and cold end with probes of Janis ST-100 cryostat in an Ar-filled glovebox. b. Close the exhaust valve of the cryostat before transferring it out from the glovebox to protect perovskite from the invasion of air. c. Measure the resistances (R) of hot end and cold end under 302, 304 and 306 K using B2912A Precision Source and temperature controller. d. Link the on-chip stripe heater to a DC stabilized power supply with external wires, and apply a voltage (V heater ) to the heater. e. Change V heater from 4 V to 10 V (step = 1 V).
i. Measure the corresponding DV using Keithley nano voltmeter.
ii. Measure the resistances of hot end and cold end by B2912A Precision Source.
Note: The TEs measurement aims to obtain the relationship between temperature difference (DT) and thermoelectric potential difference (DV), as shown in Figure 2F.
CRITICAL: Measure all Seebeck coefficients at RT in a high vacuum (< 10 -5 mbar) using Janis ST-100 in the dark.

EXPECTED OUTCOMES
The important outcomes of the present protocols are illustrated below: Conductivity measurement: The relationship of I and V 3 -V 2 of doped perovskite films can be obtained by four-point probe method (see Figure 3A), and the slope is the conductance (G) of  Figure 3B which directly demonstrates the occurrence of doping. and I DS = C i m W 2L ðV GS À V TH Þ 2 , respectively. 3 L and W are length and width of the FETs channel, respectively. The fitted range of V GS is from À40 V to À30 V. The doping ratio-dependent performance parameters are shown in Figure 4D.  at temperature of T, and R 302K is the measured resistance at 302 K. The temperature difference between hot end and cold end can also be calculated ( Figure 5B). The measured DT versus DV is shown in Figure 5C, the slope of the fitted line is the Seebeck coefficient (S = DV/DT). The power factor (PF = S 2 s) can be calculated from S ( Figure 5C) and s (Figures 3B), 4 these parameters are shown in Figure 5D.

LIMITATIONS
The conductivity of pristine (PEA) 2 SnI 4 film is too low to measure the Seebeck coefficient accurately.

TROUBLESHOOTING Problem 1
The measured conductivity is much higher than the expected value (device characterization step 4).

Potential solution
The much higher conductivity is contributed to the oxidation of Sn 2+ . The possible reasons are that SnI 2 , perovskite solution or device is stored too long.

Problem 2
The on/off ratio of transfer characteristics is much lower than the expected value (device characterization step 5).

Potential solution
The much lower on/off ratio may be caused by that the actual mass ratio differs greatly from the standard value.
Solution 1: Weigh SnI 2 and PEAI with mass ratio of 0.748 strictly.
Solution 2: Weigh SnI 4 and PEAI with mass ratio of 1.257 strictly.

Problem 3
The Seebeck coefficient of TE does not decrease with the increase of conductivity, or the resistance versus temperature of hot end and cold end ( Figure 5A) is not perfectly linear (device characterization step 6).
Potential solution Solution 1: Reconnect the probes to the electrodes.
Solution 2: Wait 20 min for the temperature to be stable before measuring resistance.

Problem 4
Imperfect perovskite film deposition may be attributed to the dirty atmosphere in glovebox (such as the existence of organic solvent vapor) or the temperature of precursor is much higher than room temperature (about 25 C here) (device fabrication step 3).
Potential solution Solution 1: Clean the glovebox with fresh Ar gas for at least 10 min.
Solution 2: Place the precursor away from heat for half an hour.
Potential solution Solution 1: If the currents of transfer or output measurements are very small, i.e., no FETs current, check the connection between probes and electrodes, and connection between probe station and Precision Source.
Solution 2: If the currents of transfer or output curves are much higher than the typical curves (as shown in Figure 4A and 4B), check that if the measurement environment is dark.

RESOURCE AVAILABILITY
Lead contact Further information and requests for resources and reagents should be directed to and will be fulfilled by the lead contact, Yuanyuan Hu (yhu@hnu.edu.cn).

Materials availability
This study did not generate new unique reagents.

Data and code availability
This study did not produce datasets/code.